Along with the recent development of facsimile machines, especially of high-speed facsimile machines, it has been required that the heat sensitive record material therefor be provided with eminent heat sensitive characteristics such as the color-forming sensitivity, humidity decolorization resistance and the like.
Various attempts have heretofore been made to improve the heat sensitivity characteristics of the heat sensitive record material. For example, Japanese Examined Patent Publication No. 17748/1974 and No. 39567/1976 propose to employ a combination of an organic acid as an acidic substance and a phenolic compound, or to use a polyvalent metal salt of a compound having an alcoholic hydroxyl group. Japanese Unexamined Patent Publication No. 11140/1974 (Japanese Examined Patent Publication No. 29945/1976) proposes to employ a copolymer of hydroxyethyl cellulose with a salt of maleic acid anhydride. Further, in Japanese Unexamined Patent Publications No. 34842/1974, No. 115554/1974, No. 149353/1975, No. 106746/1977, No. 5636/1978, No. 11036/1978 and No. 48751/1978, it is disclosed to incorporate, as a sensitizer, a nitrogen-containing organic compound such as thioacetanilide, phthalonitrile, acetamide, di-.beta.-naphtyl-p-phenylenediamine, a fatty acid amide, acetoacetic anilide, diphenylamine, benzamide or carbazole, or a heat-fluidizable material such as 2,3-di-m-tolylbutane or 4,4'-dimethylbiphenyl, or a carboxylic acid ester such as dimethylisophthalate or diphenylphthalate.
The present inventors have prepared heat sensitive record papers in accordance with the conventional methods and the methods disclosed in the above-mentioned various publications by using known electron-donating colorless dyes as a chromogenic material, and tested them for various properties required for the heat sensitive record paper. As a result of the tests, it has been found that they are inferior in the heat sensitivity characteristics, i.e. the initial color-forming temperature (Ts) under heating is high, the rising temperature coefficient (.gamma.) of the color density curve is low, or the maximum color density (D.sub.max) is low. Thus, they are practically not useful as a heat sensitive record paper for facsimile, particularly for high-speed facsimile. Further, when the thermally color-developed record papers are left to stand in an atmosphere having a relative humidity of 80 to 90% at 50.degree. to 60.degree. C. for 12 to 24 hours, the density of the color-developed chromogenic material decreases as compared with the color density immediately after the color-development, and in some cases, the color disappears completely leaving no trace of the color-developed chromogenic material. Thus, the humidity decolorization resistance is poor, so that the commercial value of such heat sensitive record paper will be impaired.